This article deals with Server Side Includes, usually called
simply SSI. In this article, I'll talk about configuring your
server to permit SSI, and introduce some basic SSI techniques
for adding dynamic content to your existing HTML pages.

In the latter part of the article, we'll talk about some of
the somewhat more advanced things that can be done with SSI,
such as conditional statements in your SSI directives.

SSI (Server Side Includes) are directives that are placed in
HTML pages, and evaluated on the server while the pages are
being served. They let you add dynamically generated content to
an existing HTML page, without having to serve the entire page
via a CGI program, or other dynamic technology.

The decision of when to use SSI, and when to have your page
entirely generated by some program, is usually a matter of how
much of the page is static, and how much needs to be
recalculated every time the page is served. SSI is a great way
to add small pieces of information, such as the current time.
But if a majority of your page is being generated at the time
that it is served, you need to look for some other
solution.

To permit SSI on your server, you must have the following
directive either in your httpd.conf file, or in a
.htaccess file:

Options +Includes

This tells Apache that you want to permit files to be parsed
for SSI directives. Note that most configurations contain
multiple Options directives
that can override each other. You will probably need to apply the
Options to the specific directory where you want SSI
enabled in order to assure that it gets evaluated last.

Not just any file is parsed for SSI directives. You have to
tell Apache which files should be parsed. There are two ways to
do this. You can tell Apache to parse any file with a
particular file extension, such as .shtml, with
the following directives:

AddType text/html .shtml
AddOutputFilter INCLUDES .shtml

One disadvantage to this approach is that if you wanted to
add SSI directives to an existing page, you would have to
change the name of that page, and all links to that page, in
order to give it a .shtml extension, so that those
directives would be executed.

XBitHack
tells Apache to parse files for SSI
directives if they have the execute bit set. So, to add SSI
directives to an existing page, rather than having to change
the file name, you would just need to make the file executable
using chmod.

chmod +x pagename.html

A brief comment about what not to do. You'll occasionally
see people recommending that you just tell Apache to parse all
.html files for SSI, so that you don't have to
mess with .shtml file names. These folks have
perhaps not heard about XBitHack. The thing to
keep in mind is that, by doing this, you're requiring that
Apache read through every single file that it sends out to
clients, even if they don't contain any SSI directives. This
can slow things down quite a bit, and is not a good idea.

Of course, on Windows, there is no such thing as an execute
bit to set, so that limits your options a little.

In its default configuration, Apache does not send the last
modified date or content length HTTP headers on SSI pages,
because these values are difficult to calculate for dynamic
content. This can prevent your document from being cached, and
result in slower perceived client performance. There are two
ways to solve this:

Use the XBitHack Full configuration. This
tells Apache to determine the last modified date by looking
only at the date of the originally requested file, ignoring
the modification date of any included files.

Use the directives provided by
mod_expires to set an explicit expiration
time on your files, thereby letting browsers and proxies
know that it is acceptable to cache them.

It is formatted like an HTML comment, so if you don't have
SSI correctly enabled, the browser will ignore it, but it will
still be visible in the HTML source. If you have SSI correctly
configured, the directive will be replaced with its
results.

The element can be one of a number of things, and we'll talk
some more about most of these in the next installment of this
series. For now, here are some examples of what you can do with
SSI

The echo element just spits out the value of a
variable. There are a number of standard variables, which
include the whole set of environment variables that are
available to CGI programs. Also, you can define your own
variables with the set element.

If you don't like the format in which the date gets printed,
you can use the config element, with a
timefmt attribute, to modify that formatting.

Earlier, we mentioned that you could use SSI to inform the
user when the document was most recently modified. However, the
actual method for doing that was left somewhat in question. The
following code, placed in your HTML document, will put such a
time stamp on your page. Of course, you will have to have SSI
correctly enabled, as discussed above.

Of course, you will need to replace the
ssi.shtml with the actual name of the file that
you're referring to. This can be inconvenient if you're just
looking for a generic piece of code that you can paste into any
file, so you probably want to use the
LAST_MODIFIED variable instead:

If you are managing any site that is more than a few pages,
you may find that making changes to all those pages can be a
real pain, particularly if you are trying to maintain some kind
of standard look across all those pages.

Using an include file for a header and/or a footer can
reduce the burden of these updates. You just have to make one
footer file, and then include it into each page with the
include SSI command. The include
element can determine what file to include with either the
file attribute, or the virtual
attribute. The file attribute is a file path,
relative to the current directory. That means that it
cannot be an absolute file path (starting with /), nor can it
contain ../ as part of that path. The virtual
attribute is probably more useful, and should specify a URL
relative to the document being served. It can start with a /,
but must be on the same server as the file being served.

<!--#include virtual="/footer.html" -->

I'll frequently combine the last two things, putting a
LAST_MODIFIED directive inside a footer file to be
included. SSI directives can be contained in the included file,
and includes can be nested - that is, the included file can
include another file, and so on.

In addition to being able to config the time
format, you can also config two other things.

Usually, when something goes wrong with your SSI directive,
you get the message

[an error occurred while processing this directive]

If you want to change that message to something else, you
can do so with the errmsg attribute to the
config element:

<!--#config errmsg="[It appears that you don't know how to use SSI]" -->

Hopefully, end users will never see this message, because
you will have resolved all the problems with your SSI
directives before your site goes live. (Right?)

And you can config the format in which file
sizes are returned with the sizefmt attribute. You
can specify bytes for a full count in bytes, or
abbrev for an abbreviated number in Kb or Mb, as
appropriate.

I expect that I'll have an article some time in the coming
months about using SSI with small CGI programs. For now, here's
something else that you can do with the exec
element. You can actually have SSI execute a command using the
shell (/bin/sh, to be precise - or the DOS shell,
if you're on Win32). The following, for example, will give you
a directory listing.

<pre>
<!--#exec cmd="ls" -->
</pre>

or, on Windows

<pre>
<!--#exec cmd="dir" -->
</pre>

You might notice some strange formatting with this directive
on Windows, because the output from dir contains
the string ``<dir>'' in it, which confuses
browsers.

Note that this feature is exceedingly dangerous, as it will
execute whatever code happens to be embedded in the
exec tag. If you have any situation where users
can edit content on your web pages, such as with a
``guestbook'', for example, make sure that you have this
feature disabled. You can allow SSI, but not the
exec feature, with the IncludesNOEXEC
argument to the Options directive.

Using the set directive, you can set variables
for later use. We'll need this later in the discussion, so
we'll talk about it here. The syntax of this is as follows:

<!--#set var="name" value="Rich" -->

In addition to merely setting values literally like that, you
can use any other variable, including environment variables or the variables
discussed above (like LAST_MODIFIED, for example) to
give values to your variables. You will specify that something is
a variable, rather than a literal string, by using the dollar sign
($) before the name of the variable.

<!--#set var="modified" value="$LAST_MODIFIED" -->

To put a literal dollar sign into the value of your
variable, you need to escape the dollar sign with a
backslash.

<!--#set var="cost" value="\$100" -->

Finally, if you want to put a variable in the midst of a
longer string, and there's a chance that the name of the
variable will run up against some other characters, and thus be
confused with those characters, you can place the name of the
variable in braces, to remove this confusion. (It's hard to
come up with a really good example of this, but hopefully
you'll get the point.)

Now that we have variables, and are able to set and compare
their values, we can use them to express conditionals. This
lets SSI be a tiny programming language of sorts.
mod_include provides an if,
elif, else, endif
structure for building conditional statements. This allows you
to effectively generate multiple logical pages out of one
actual page.

A test_condition can be any sort of logical
comparison - either comparing values to one another, or testing
the ``truth'' of a particular value. (A given string is true if
it is nonempty.) For a full list of the comparison operators
available to you, see the mod_include
documentation. Here are some examples of how one might use this
construct.

Not that I have anything against IE on Macs - I just
struggled for a few hours last week trying to get some
JavaScript working on IE on a Mac, when it was working
everywhere else. The above was the interim workaround.

Any other variable (either ones that you define, or normal
environment variables) can be used in conditional statements.
With Apache's ability to set environment variables with the
SetEnvIf directives, and other related directives,
this functionality can let you do some pretty involved dynamic
stuff without ever resorting to CGI.

SSI is certainly not a replacement for CGI, or other
technologies used for generating dynamic web pages. But it is a
great way to add small amounts of dynamic content to pages,
without doing a lot of extra work.

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